The formation conditions of the axial current of the DC motor are the axial voltage and the loop. In the motor, where the design and operating conditions are normal, the voltage difference between the two ends of the shaft is only small, and it is not enough to cause damage due to oil film or bearing insulation treatment. If there is a problem in a certain link and the shaft voltage exceeds the limit value, the original insulation may be broken, and a loop will be formed in the shaft, bearing inner ring, bearing outer ring, and bearing cavity. Therefore, the position of the shaft bearing and the surface of the bearing inner ring will be caused by arc discharge Produce small and deep circular corrosion spots or strip-shaped arcs, which are enough to burn the journal and bearing bush in severe cases. Then, how do we solve the problem that the DC motor has a large starting current?
From the perspective of the starting principle of the DC motor and the principle of motor rotation, when the induction DC motor is in a stopped state, from the point of view of electromagnetics, it is like a transformer. The stator winding connected to the power supply is equivalent to the primary winding of the transformer, and the closed rotor winding is equivalent to the short-circuited secondary winding of the transformer. There is no electrical connection between the stator winding and the rotor winding, only a magnetic connection, and the magnetic flux is closed through the stator, the air gap and the rotor core.
In the closing moments, DC motor due to the inertia of the rotor has not yet started, the rotating magnetic field - a relatively large cutting speed synchronous speed cutting rotor windings, the rotor winding is likely to induce a high electric potential. Therefore, a large current flows through the rotor conductor, generating magnetic energy to cancel the stator magnetic field, just as the secondary magnetic flux of a transformer cancels the primary magnetic flux. On the other hand, in order to maintain the original magnetic flux compatible with the current power supply voltage, the stator automatically increases the current.
